As improvements have been continually made to television receivers, the number of adjustments which must be made by the viewer have been substantially reduced. One adjustment, however, which still remains is a fine tuning adjustment. Such an adjustment is required even with receivers having an automatic fine tuning (AFT or AFC) system in them. With respect to the VHF channels, the fine tuning adjustment generally is made only when the receiver is first put into operation and then infrequently afterwards as components of the receiver age. For the UHF channels, however, a fine tuning adjustment generally is required each time the UHF station is tuned in by the viewer. This is annoying, and it is desirable to eliminate the need for such a fine tuning adjustment.
Wide-band automatic fine tuning systems, that is, systems capable of properly tuning a receiver having a rough tuning adjustment which is off by as much as .+-.2 megahertz have been attempted in the past. Generally such wide-band AFT systems require a very stable oscillator at the IF frequency. This substantially increases the cost of the system and adds considerably to the final cost of the television receiver. Because of this, systems requiring such highly stable oscillators have not met with much practical success.
Some television receivers also have been equipped with signal seeking tuning circuits. In some respects these systems resemble a wide-band AFT system inasmuch as the receiver uses a motor to scan the transmission spectrum and to stop the scanning, and therefore tuning of the receiver, in response to the appearance of a video carrier, a sound carrier and generally the horizontal synchronizing signals in a proper relationship to prevent improper lockup of the receiver on the wrong signals. Such systems, however, operate in one direction only, even for systems which are referred to as bi-directional signal seeking systems. The bi-directional systems scan in one direction or the other at the option of the user, or upon completion of a scan in a first direction, automatically reverse to scan the spectrum in the opposite direction.
If a signal seeking system, however, should over-shoot the proper tuning for a station for any reason, the tuning direction does not reverse to effect proper tuning. The same result occurs if the station should subsequently drift off the proper frequency in the reverse direction from the scan direction. The only way such signal-seeking tuning systems for television receivers have of subsequently properly returning the receiver in such situations is to reverse the scan direction either manually or by completing the scan of the entire frequency spectrum in the first direction, whereupon reversal takes place.
It is desirable to provide a wide-band AFT system for a television receiver which has a pull-in range wide enough to eliminate the need for viewer fine tuning adjustments once the gross channel settings for tuning the receiver have been made. In addition, it is desirable to provide for such a wide-band AFT system without requiring the use of highly stable oscillators or other expensive components.